Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections of the object by using information on how the waves are changed upon reflection.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. In order to increase performance, variants of this technique have been developed using two wavelengths in so-called dual band OCT systems.
In parallel Fourier domain OCT (FD-OCT) techniques have been developed. One example is time-encoded OCT, which uses a wavelength swept source and a single detector; it is sometimes referred to as time-encoded FD-OCT (TEFD-OCT) or swept source OCT. Another example is spectrum encoded OCT, which uses a broadband source and spectrally resolving detector system and is sometimes referred to as spectrum-encoded FD-OCT or SEFD-OCT.
Interestingly, these three OCT techniques, TD-OCT, TEFD-OCT, and SEFD-OCT, parallel the three spectrometer architectures of Fourier Transform spectrometers, tunable laser spectrometers, and dispersive grating with detector array spectrometers.
These various OCT techniques offer different performance characteristics. FD-OCT has advantages over TD-OCT in speed and signal-to-noise ratio (SNR). Of the two FD-OCT techniques, swept-source OCT or TEFD-OCT has distinct advantages over spectrum-encoded FD-OCT or SEFD-OCT because of its capability of balanced and polarization diversity detection; it has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for SEFD-OCT, are not available.
TEFD-OCT or swept source OCT has advantages over SEFD-OCT in some additional respects. The spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its tuning speed and accuracy.
The swept sources for TEFD-OCT systems have been typically tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. The typical tunable laser is constructed from a gain medium, such as a semiconductor optical amplifier (SOA), and a tunable filter such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest speed TEFD-OCT lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia. This highly integrated design allows for a short laser cavity that keeps the round-trip optical travel times within the laser short so that the laser is fundamentally capable of high speed tuning Secondly, the use of micro-electro-mechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low mass high mechanical resonant frequency deflectable MEMS membranes that can be tuned quickly.
Another class of swept sources that have the potential to avoid inherent drawbacks of tunable lasers is filtered amplified spontaneous emission (ASE) sources that combine a broadband light source, typically a source that generates light by ASE, with tunable filters and amplifiers.
Some of the highest speed devices based on filtered ASE sources are described in U.S. Pat. No. 7,061,618 B2, entitled Integrated Spectroscopy System, by W. Atia, D. Flanders P. Kotidis, and M. Kuznetsov, which describes spectroscopy engines for diffuse reflectance spectroscopy and other spectroscopic applications. A number of variants of the filtered ASE swept source are described, including amplified versions and versions with tracking filters.
More recently Eigenwillig, et al. have proposed a variant configuration of the filtered ASE source in an article entitled “Wavelength swept ASE source”, Conference Title: Optical Coherence Tomography and Coherence Techniques IV, Munich, Germany, Proc. SPIE 7372, 737200 (Jul. 13, 2009). The article describes an SOA functioning both as an ASE source and first amplification stage. Two Fabry-Perot tunable filters are used in a primary-tracking filter arrangement, which are followed by a second SOA amplification stage. Also, U.S. patent application Ser. No. 12/553,295, filed on Sep. 3, 2009, now U.S. Pat. Appl. Pub. No. US 2011/0051148 A1, entitled Filtered ASE Swept Source for OCT Medical Imaging, by D. Flanders, W. Atia, and M. Kuznetsov, which is incorporated herein in its entirety by this reference, lays out various integrated, high speed filtered ASE swept source configurations.